(mRNA), ribosomal RNA (rRNA), or transfer RNA (tRNA). In prokaryotic cells all
these different RNA types are synthesized by the same polymerase. This enzyme has
an affinity to a specific DNA sequence, the promoter, that also indicates the first base
to be copied. During initiation of transcription, the RNA polymerase binds to the pro-
moter with a high affinity that is supported by further initiation factors. Complete for-
mation of the initiation complex causes the DNA to unwind in the promoter region.
Now the enzyme is ready to add the first RNA nucleoside triphosphate to the template
strand of the opened DNA double strand. In the subsequent elongation phase, the
RNA polymerase moves along the unwinding DNA and extends the newly developing
mRNA continuously with nucleotides complementary to the template strand. During
this phase a moving transient, double-stranded RNA-DNA hybrid is established. As
the polymerase moves along, the DNA rewinds again just behind it. As RNA synthesis
always proceeds in 5'?3' direction, only one of the DNA chains acts as a template,
the so-called antisense (–) strand. The other one, the sense (+) strand, has the same se-
quence as the transcribed RNA, except for the thymine nucleotides that are replaced
by uracil nucleotides in RNA. As much as the promoter is responsible for initiation of
transcription, the terminator – another specific DNA sequence – is responsible for its
termination. For the bacterium E. coli two different termination mechanisms are de-
scribed: the Rho-independent and Rho-dependent terminations. In Rho-independent
termination, the transcribed terminator region shows two short GC-rich and self-com-
plementary sequences that can bind to each other and thus form a so-called hairpin
structure. This motif is followed by a block of uracil residues that bind the comple-
mentary adenine residues of the DNA only weakly. Presumably, this RNA structure
causes the RNA polymerase to terminate and release the RNA. In Rho-dependent ter-
mination, a protein – the Rho factor – can bind the newly synthesized RNA near the
terminator and mediate the RNA release. Termination in eukaryotic cells shows both,
similarities to and differences from the mechanisms found in bacteria.
2.4.2
Processing of the mRNA
In eukaryotic cells the primary mRNA transcript (precursor mRNA or pre-mRNA) is
further processed before being exported into the cytosol and entering translation (Fig.
2.11 2). The protein-coding sequence lies internally in the mRNA and is flanked on
both sides by nucleotides that are not translated. During processing, a so-called 5' cap
is attached to the flanking 5' untranslated region (5' UTR, about 10 to 200 nucleotides)
preceding, or lying upstream of, the coding sequence. This 5' cap consists of three nu-
cleotides that are further modified. The 3' untranslated region (3' UTR) of most
mRNAs is also modified after transcription by addition of a series of about 30 to 200
47
2.4 Expression of Genes
3 Fig. 2.11 Gene expression in eukaryotic cells
comprises several steps from the DNA to the
mature protein at its final destination. This in-
volves the 1 transcription of the gene, 2 spli-
cing and processing of the pre-mRNA, 3 export
of the mature mRNA into the cytosol, 4 trans-
lation of the genetic code into a protein, and
5–0 several steps of sorting and modification.
More details are given in the text.